Write signature command

ABSTRACT

In one aspect, a method includes generating a write signature command. The write signature command is configured to write a signature to an offset in a storage array without data and to enable the storage array to write the data with the same signature to a volume if the data is available at the storage array.

BACKGROUND

Computer data is vital to today's organizations, and a significant partof protection against disasters is focused on data protection. Assolid-state memory has advanced to the point where cost of memory hasbecome a relatively insignificant factor, organizations can afford tooperate with systems that store and process terabytes of data.

Conventional data protection systems include tape backup drives, forstoring organizational production site data on a periodic basis. Suchsystems suffer from several drawbacks. First, they require a systemshutdown during backup, since the data being backed up cannot be usedduring the backup operation. Second, they limit the points in time towhich the production site can recover. For example, if data is backed upon a daily basis, there may be several hours of lost data in the eventof a disaster. Third, the data recovery process itself takes a longtime.

Another conventional data protection system uses data replication, bycreating a copy of the organization's production site data on asecondary backup storage system, and updating the backup with changes.The backup storage system may be situated in the same physical locationas the production storage system, or in a physically remote location.Data replication systems generally operate either at the applicationlevel, at the file system level, or at the data block level.

Current data protection systems try to provide continuous dataprotection, which enable the organization to roll back to any specifiedpoint in time within a recent history. Continuous data protectionsystems aim to satisfy two conflicting objectives, as best as possible;namely, (i) minimize the down time, in which the organization productionsite data is unavailable, during a recovery, and (ii) enable recovery asclose as possible to any specified point in time within a recenthistory.

Continuous data protection typically uses a technology referred to as“journaling,” whereby a log is kept of changes made to the backupstorage. During a recovery, the journal entries serve as successive“undo” information, enabling rollback of the backup storage to previouspoints in time. Journaling was first implemented in database systems,and was later extended to broader data protection.

One challenge to continuous data protection is the ability of a backupsite to keep pace with the data transactions of a production site,without slowing down the production site. The overhead of journalinginherently requires several data transactions at the backup site foreach data transaction at the production site. As such, when datatransactions occur at a high rate at the production site, the backupsite may not be able to finish backing up one data transaction beforethe next production site data transaction occurs. If the production siteis not forced to slow down, then necessarily a backlog of un-logged datatransactions may build up at the backup site. Without being able tosatisfactorily adapt dynamically to changing data transaction rates, acontinuous data protection system chokes and eventually forces theproduction site to shut down.

SUMMARY

In one aspect, a method includes generating a write signature command.The write signature command is configured to write a signature to anoffset in a storage array without data and to enable the storage arrayto write the data with the same signature to a volume if the data isavailable at the storage array.

In another aspect, an article includes a non-transitory machine-readablemedium that stores executable instructions. The instructions cause amachine to generate a write signature command. The write signaturecommand is configured to write a signature to an offset in a storagearray without data and to enable the storage array to write the datawith the same signature to a volume if the data is available at thestorage array.

In a further aspect, an apparatus includes circuitry to generate a writesignature command. The write signature command configured to write asignature to an offset in a storage array without data and to enable thestorage array to write the data with the same signature to a volume ifthe data is available at the storage array.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example of a data protection system.

FIG. 2 is a block diagram of an example of a write transaction for ajournal.

FIG. 3 is an example of a read signature command.

FIG. 4 is a block diagram of an example of a production site executing aread signature command and sending the signature to a replication site.

FIG. 5 is a flowchart of an example of a process to use read signatures.

FIG. 6 is a block diagram of an example of an alternative example of aproduction site executing a read signature command and sending thesignature to a replication site.

FIG. 7 is a flowchart of another example of a process to use readsignatures.

FIG. 8 is a flowchart of a further example of a process to use readsignatures.

FIG. 9 is a block diagram of another example of a production site with abitmap to execute a read signature command and to send the signature toa replication site with a bitmap.

FIG. 10 is a flowchart of an example of a process to use read signatureswith bitmaps.

FIG. 11 is a block diagram of another example of a production site withmultiple read signatures and a replication site with multiple readsignatures.

FIG. 12 is a block diagram of a storage array including a hash table.

FIG. 13A is a flowchart of an example of a process to update signaturessynchronously.

FIG. 13B is a flowchart of an example of a process to update signaturesasynchronously.

FIG. 14A is a flowchart of an example of a process to handle writesignatures in a synchronous mode.

FIG. 14B is a flowchart of an example of a process to handle writesignatures in an asynchronous mode.

FIG. 15 is a block diagram of an external data protection applianceincluding a hash table.

FIG. 16 is an example of a write signature command.

FIG. 17A is a flowchart of an example of a process to synchronizeproduction and replication storage.

FIG. 17B is a flowchart of an example of a process to use a snapshotshipping mode.

FIG. 18 is a block diagram of an example of a production site executinga write signature command and sending the signature to a replicationsite.

FIG. 19 is a flowchart of an example of a process to leverage a storagesystem supporting write signature as a device that deduplicated data.

FIG. 20 is a computer on which any of the processes of FIGS. 14, 15A,15B and 17 may be implemented.

DETAILED DESCRIPTION

In one example, a write signature command may be added to a storagedevice. In certain examples, the write signature command may have a LUN,Logical Block Address (LBA), and number of blocks as parameters and mayreturn a status (e.g., success, mismatched signature, or a temporaryfailure). In some examples, using the signatures may enable fullsilvering to be performed about 500 times faster than traditionalsilvering, since there is almost no data transfer. In still furtherexamples, a periodic consistency and verification check may be run onvolumes and their replicas.

The following definitions are employed throughout the specification andclaims.

BACKUP SITE—may be a facility where replicated production site data isstored; the backup site may be located in a remote site or at the samelocation as the production site.

CLONE—a clone may be a copy or clone of the image or images, drive ordrives of a first location at a second location.

DELTA MARKING STREAM—may mean the tracking of the delta between theproduction and replication site, which may contain the metadata ofchanged locations, the delta marking stream may be kept persistently onthe journal at the production site of the replication, based on thedelta marking data the DPA knows which locations are different betweenthe production and the replication and transfers them to the replicationto make both sites identical.

DPA—may be Data Protection Appliance a computer or a cluster ofcomputers, or a set of processes that serve as a data protectionappliance, responsible for data protection services including inter aliadata replication of a storage system, and journaling of I/O requestsissued by a host computer to the storage system.

RPA—may be replication protection appliance, is another name for DPA.

HOST—may be at least one computer or networks of computers that runs atleast one data processing application that issues I/O requests to one ormore storage systems; a host is an initiator with a SAN.

HOST DEVICE—may be an internal interface in a host, to a logical storageunit.

IMAGE—may be a copy of a logical storage unit at a specific point intime.

INITIATOR—may be a node in a SAN that issues I/O requests.

JOURNAL—may be a record of write transactions issued to a storagesystem; used to maintain a duplicate storage system, and to rollback theduplicate storage system to a previous point in time.

LOGICAL UNIT—may be a logical entity provided by a storage system foraccessing data from the storage system.

LUN—may be a logical unit number for identifying a logical unit.

PHYSICAL STORAGE UNIT—may be a physical entity, such as a disk or anarray of disks, for storing data in storage locations that can beaccessed by address.

PRODUCTION SITE—may be a facility where one or more host computers rundata processing applications that write data to a storage system andread data from the storage system.

SAN—may be a storage area network of nodes that send and receive I/O andother requests, each node in the network being an initiator or a target,or both an initiator and a target.

SOURCE SIDE—may be a transmitter of data within a data replicationworkflow, during normal operation a production site is the source side;and during data recovery a backup site is the source side.

SNAPSHOT—a Snapshot may refer to differential representations of animage, i.e., the snapshot may have pointers to the original volume, andmay point to log volumes for changed locations. Snapshots may becombined into a snapshot array, which may represent different imagesover a time period.

STORAGE SYSTEM—may be a SAN entity that provides multiple logical unitsfor access by multiple SAN initiators.

TARGET—may be a node in a SAN that replies to I/O requests.

TARGET SIDE—may be a receiver of data within a data replicationworkflow; during normal operation a back site is the target side, andduring data recovery a production site is the target side.

WAN—may be a wide area network that connects local networks and enablesthem to communicate with one another, such as the Internet.

SPLITTER/PROTECTION AGENT—may be an agent running either on a productionhost a switch or a storage array which can intercept IO and split themto a DPA and to the storage array, fail IO redirect IO or do any othermanipulation to the IO.

VIRTUAL VOLUME—may be a volume which is exposed to host by avirtualization layer, the virtual volume may be spanned across more thanone site.

DISTRIBUTED MIRROR—may be a mirror of a volume across distance, eithermetro or geo, which is accessible at all sites.

BLOCK VIRTUALIZATION—may be a layer, which takes backend storage volumesand by slicing concatenation and striping create a new set of volumes,which serve as base volumes or devices in the virtualization layer.

MARKING ON SPLITTER—may be a mode in a splitter where intercepted IOsare not split to an appliance and the storage, but changes (metadata)are tracked in a list and/or a bitmap and I/O is immediately sent todown the IO stack.

FAIL ALL MODE—may be a mode of a volume in the splitter where all writeand read IOs intercepted by the splitter are failed to the host, butother SCSI commands like read capacity are served.

GLOBAL FAIL ALL MODE—may be a mode of a volume in the virtual layerwhere all write and read IOs virtual layer are failed to the host, butother SCSI commands like read capacity are served.

LOGGED ACCESS—may be an access method provided by the appliance and thesplitter, in which the appliance rolls the volumes of the consistencygroup to the point in time the user requested and let the host accessthe volumes in a copy on first write base.

VIRTUAL ACCESS—may be an access method provided by the appliance and thesplitter, in which the appliance exposes a virtual volume from aspecific point in time to the host, the data for the virtual volume ispartially stored on the remote copy and partially stored on the journal.

CDP—Continuous Data Protection, may refer to a full replication of avolume or a set of volumes along with a journal which allows any pointin time access, the CDP copy is at the same site, and maybe the samestorage array of the production site.

CRR—Continuous Remote Replication may refer to a full replication of avolume or a set of volumes along with a journal which allows any pointin time access at a site remote to the production volume and on aseparate storage array.

As used herein, the term storage medium may refer to one or more storagemediums such as a hard drive, a combination of hard drives, flashstorage, combinations of flash storage, combinations of hard drives,flash, and other storage devices, and other types and combinations ofcomputer readable storage mediums including those yet to be conceived. Astorage medium may also refer both physical and logical storage mediumsand may include multiple level of virtual to physical mappings and maybe or include an image or disk image.

A description of journaling and some techniques associated withjournaling may be described in the patent titled METHODS AND APPARATUSFOR OPTIMAL JOURNALING FOR CONTINUOUS DATA REPLICATION and with U.S.Pat. No. 7,516,287, which is hereby incorporated by reference.

A discussion of image access may be found in U.S. patent applicationSer. No. 12/969,903 entitled “DYNAMIC LUN RESIZING IN A REPLICATIONENVIRONMENT” filed on Dec. 16, 2010 assigned to EMC Corp., which ishereby incorporated by reference.

Five State Journaling

Reference is now made to FIG. 1, which is a simplified illustration of adata protection system 100. Shown in FIG. 1 are two sites; Site I, whichis a production site, on the right, and Site II, which is a backup site,on the left. Under normal operation the production site is the sourceside of system 100, and the backup site is the target side of thesystem. The backup site is responsible for replicating production sitedata. Additionally, the backup site enables rollback of Site I data toan earlier pointing time, which may be used in the event of datacorruption of a disaster, or alternatively in order to view or to accessdata from an earlier point in time.

During normal operations, the direction of replicate data flow goes fromsource side to target side. It is possible, however, for a user toreverse the direction of replicate data flow, in which case Site Istarts to behave as a target backup site, and Site II starts to behaveas a source production site. Such change of replication direction isreferred to as a “failover”. A failover may be performed in the event ofa disaster at the production site, or for other reasons. In some dataarchitectures, Site I or Site II behaves as a production site for aportion of stored data, and behaves simultaneously as a backup site foranother portion of stored data. In some data architectures, a portion ofstored data is replicated to a backup site, and another portion is not.

The production site and the backup site may be remote from one another,or they may both be situated at a common site, local to one another.Local data protection has the advantage of minimizing data lag betweentarget and source, and remote data protection has the advantage is beingrobust in the event that a disaster occurs at the source side.

The source and target sides communicate via a wide area network (WAN)128, although other types of networks are also adaptable for use withexamples described herein.

In one example, each side of system 100 includes three major componentscoupled via a storage area network (SAN); namely, (i) a storage system,(ii) a host computer, and (iii) a data protection appliance (DPA).Specifically with reference to FIG. 1, the source side SAN includes asource host computer 104, a source storage system 108, and a source DPA112. Similarly, the target side SAN includes a target host computer 116,a target storage system 120, and a target DPA 124.

Generally, a SAN includes one or more devices, referred to as “nodes”. Anode in a SAN may be an “initiator” or a “target”, or both. An initiatornode is a device that is able to initiate requests to one or more otherdevices; and a target node is a device that is able to reply torequests, such as SCSI commands, sent by an initiator node. A SAN mayalso include network switches, such as fiber channel switches. Thecommunication links between each host computer and its correspondingstorage system may be any appropriate medium suitable for data transfer,such as fiber communication channel links.

In one example, the host communicates with its corresponding storagesystem using small computer system interface (SCSI) commands.

System 100 includes source storage system 108 and target storage system120. Each storage system includes physical storage units for storingdata, such as disks or arrays of disks. Typically, storage systems 108and 120 are target nodes. In order to enable initiators to send requeststo storage system 108, storage system 108 exposes one or more logicalunits (LU) to which commands are issued. Thus, storage systems 108 and120 are SAN entities that provide multiple logical units for access bymultiple SAN initiators.

Logical units are a logical entity provided by a storage system, foraccessing data stored in the storage system. A logical unit isidentified by a unique logical unit number (LUN). In one example,storage system 108 exposes a logical unit 136, designated as LU A, andstorage system 120 exposes a logical unit 156, designated as LU B.

In one example, LU B is used for replicating LU A. As such, LU B isgenerated as a copy of LU A. In one example, LU B is configured so thatits size is identical to the size of LU A. Thus for LU A, storage system120 serves as a backup for source side storage system 108.Alternatively, as mentioned hereinabove, some logical units of storagesystem 120 may be used to back up logical units of storage system 108,and other logical units of storage system 120 may be used for otherpurposes. Moreover, in some examples, there is symmetric replicationwhereby some logical units of storage system 108 are used forreplicating logical units of storage system 120, and other logical unitsof storage system 120 are used for replicating other logical units ofstorage system 108.

System 100 includes a source side host computer 104 and a target sidehost computer 116. A host computer may be one computer, or a pluralityof computers, or a network of distributed computers, each computer mayinclude inter alia a conventional CPU, volatile and non-volatile memory,a data bus, an I/O interface, a display interface and a networkinterface. Generally a host computer runs at least one data processingapplication, such as a database application and an e-mail server.

Generally, an operating system of a host computer creates a host devicefor each logical unit exposed by a storage system in the host computerSAN. A host device is a logical entity in a host computer, through whicha host computer may access a logical unit. In one example, host device104 identifies LU A and generates a corresponding host device 140,designated as Device A, through which it can access LU A. Similarly,host computer 116 identifies LU B and generates a corresponding device160, designated as Device B.

In one example, in the course of continuous operation, host computer 104is a SAN initiator that issues I/O requests (write/read operations)through host device 140 to LU A using, for example, SCSI commands. Suchrequests are generally transmitted to LU A with an address that includesa specific device identifier, an offset within the device, and a datasize. Offsets are generally aligned to 512 byte blocks. The average sizeof a write operation issued by host computer 104 may be, for example, 10kilobytes (KB); i.e., 20 blocks. For an I/O rate of 50 megabytes (MB)per second, this corresponds to approximately 5,000 write transactionsper second.

System 100 includes two data protection appliances, a source side DPA112 and a target side DPA 124. A DPA performs various data protectionservices, such as data replication of a storage system, and journalingof I/O requests issued by a host computer to source side storage systemdata. As explained in detail herein, when acting as a target side DPA, aDPA may also enable rollback of data to an earlier point in time, andprocessing of rolled back data at the target site. Each DPA 112 and 124is a computer that includes inter alia one or more conventional CPUs andinternal memory.

For additional safety precaution, each DPA is a cluster of suchcomputers. Use of a cluster ensures that if a DPA computer is down, thenthe DPA functionality switches over to another computer. The DPAcomputers within a DPA cluster communicate with one another using atleast one communication link suitable for data transfer via fiberchannel or IP based protocols, or such other transfer protocol. Onecomputer from the DPA cluster serves as the DPA leader. The DPA clusterleader coordinates between the computers in the cluster, and may alsoperform other tasks that require coordination between the computers,such as load balancing.

In the architecture illustrated in FIG. 1, DPA 112 and DPA 124 arestandalone devices integrated within a SAN. Alternatively, each of DPA112 and DPA 124 may be integrated into storage system 108 and storagesystem 120, respectively, or integrated into host computer 104 and hostcomputer 116, respectively. Both DPAs communicate with their respectivehost computers through communication lines such as fiber channels using,for example, SCSI commands.

In one example, DPAs 112 and 124 are configured to act as initiators inthe SAN; i.e., they can issue I/O requests using, for example, SCSIcommands, to access logical units on their respective storage systems.DPA 112 and DPA 124 are also configured with the necessary functionalityto act as targets; i.e., to reply to I/O requests, such as SCSIcommands, issued by other initiators in the SAN, including inter aliatheir respective host computers 104 and 116. Being target nodes, DPA 112and DPA 124 may dynamically expose or remove one or more logical units.

As described hereinabove, Site I and Site II may each behavesimultaneously as a production site and a backup site for differentlogical units. As such, DPA 112 and DPA 124 may each behave as a sourceDPA for some logical units, and as a target DPA for other logical units,at the same time.

In one example, host computer 104 and host computer 116 includeprotection agents 144 and 164, respectively. Protection agents 144 and164 intercept SCSI commands issued by their respective host computers,via host devices to logical units that are accessible to the hostcomputers. In one example, a protection agent (also called herein asplitter) may act on an intercepted SCSI commands issued to a logicalunit, in one of the following ways: Send the SCSI commands to itsintended logical unit; Redirect the SCSI command to another logicalunit; Split the SCSI command by sending it first to the respective DPA;After the DPA returns an acknowledgement, send the SCSI command to itsintended logical unit; Fail a SCSI command by returning an error returncode; and Delay a SCSI command by not returning an acknowledgement tothe respective host computer.

The protection agent may handle different SCSI commands, differently,according to the type of the command. For example, a SCSI commandinquiring about the size of a certain logical unit may be sent directlyto that logical unit, while a SCSI write command may be split and sentfirst to a DPA associated with the agent. The protection agent may alsochange its behavior for handling SCSI commands, for example as a resultof an instruction received from the DPA.

Specifically, the behavior of the protection agent for a certain hostdevice generally corresponds to the behavior of its associated DPA withrespect to the logical unit of the host device. When a DPA behaves as asource site DPA for a certain logical unit, then during normal course ofoperation, the associated protection agent splits I/O requests issued bya host computer to the host device corresponding to that logical unit.Similarly, when a DPA behaves as a target device for a certain logicalunit, then during normal course of operation, the associated protectionagent fails I/O requests issued by host computer to the host devicecorresponding to that logical unit.

Communication between protection agents and their respective DPAs mayuse any protocol suitable for data transfer within a SAN, such as fiberchannel, or SCSI over fiber channel. The communication may be direct, orvia a logical unit exposed by the DPA. In one example, protection agentscommunicate with their respective DPAs by sending SCSI commands overfiber channel.

In one example, protection agents 144 and 164 are drivers located intheir respective host computers 104 and 116. Alternatively, a protectionagent may also be located in a fiber channel switch, or in any otherdevice situated in a data path between a host computer and a storagesystem.

What follows is a detailed description of system behavior under normalproduction mode, and under recovery mode.

In one example, in a production mode DPA 112 acts as a source site DPAfor LU A. Thus, the protection agent 144 is configured to act as asource side protection agent; i.e., as a splitter for host device A.Specifically, the protection agent 144 replicates SCSI I/O requests. Areplicated SCSI I/O request is sent to DPA 112. After receiving anacknowledgement from DPA 124, protection agent 144 then sends the SCSII/O request to LU A. Only after receiving a second acknowledgement fromstorage system 108 may host computer 104 initiate another I/O request.

When DPA 112 receives a replicated SCSI write request from a protectionagent 144, DPA 112 transmits certain I/O information characterizing thewrite request, packaged as a “write transaction”, over WAN 128 to DPA124 on the target side, for journaling and for incorporation withintarget storage system 120.

DPA 112 may send its write transactions to DPA 124 using a variety ofmodes of transmission, including inter alia (i) a synchronous mode, (ii)an asynchronous mode, and (iii) a snapshot mode. In synchronous mode,DPA 112 sends each write transaction to DPA 124, receives back anacknowledgement from DPA 124, and in turns sends an acknowledgement backto protection agent 144. Protection agent 144 waits until receipt ofsuch acknowledgement before sending the SCSI write request to LU A.

In asynchronous mode, DPA 112 sends an acknowledgement to protectionagent 144 upon receipt of each I/O request, before receiving anacknowledgement back from DPA 124.

In snapshot mode, DPA 112 receives several I/O requests and combinesthem into an aggregate “snapshot” of all write activity performed in themultiple I/O requests, and sends the snapshot to DPA 124, for journalingand for incorporation in target storage system 120. In snapshot mode DPA112 also sends an acknowledgement to protection agent 144 upon receiptof each I/O request, before receiving an acknowledgement back from DPA124.

For the sake of clarity, the ensuing discussion assumes that informationis transmitted at write-by-write granularity.

While in production mode, DPA 124 receives replicated data of LU A fromDPA 112, and performs journaling and writing to storage system 120. Whenapplying write operations to storage system 120, DPA 124 acts as aninitiator, and sends SCSI commands to LU B.

During a recovery mode, DPA 124 undoes the write transactions in thejournal, so as to restore storage system 120 to the state it was at, atan earlier time.

As described hereinabove, in one example, LU B is used as a backup of LUA. As such, during normal production mode, while data written to LU A byhost computer 104 is replicated from LU A to LU B, host computer 116should not be sending I/O requests to LU B. To prevent such I/O requestsfrom being sent, protection agent 164 acts as a target site protectionagent for host Device B and fails I/O requests sent from host computer116 to LU B through host Device B.

In one example, target storage system 120 exposes a logical unit 176,referred to as a “journal LU”, for maintaining a history of writetransactions made to LU B, referred to as a “journal”. Alternatively,journal LU 176 may be striped over several logical units, or may residewithin all of or a portion of another logical unit. DPA 124 includes ajournal processor 180 for managing the journal.

Journal processor 180 functions generally to manage the journal entriesof LU B. Specifically, journal processor 180 (i) enters writetransactions received by DPA 124 from DPA 112 into the journal, bywriting them into the journal LU, (ii) applies the journal transactionsto LU B, and (iii) updates the journal entries in the journal LU withundo information and removes already-applied transactions from thejournal. As described below, with reference to FIGS. 2 and 3A to 3D,journal entries include four streams, two of which are written whenwrite transaction are entered into the journal, and two of which arewritten when write transaction are applied and removed from the journal.

Reference is now made to FIG. 2, which is a simplified illustration of awrite transaction 200 for a journal, in one example. The journal may beused to provide an adaptor for access to storage 120 at the state it wasin at any specified point in time. Since the journal contains the “undo”information necessary to rollback storage system 120, data that wasstored in specific memory locations at the specified point in time maybe obtained by undoing write transactions that occurred subsequent tosuch point in time.

Write transaction 200 generally includes the following fields: one ormore identifiers; a time stamp, which is the date & time at which thetransaction was received by source side DPA 112; a write size, which isthe size of the data block; a location in journal LU 176 where the datais entered; a location in LU B where the data is to be written; and thedata itself.

Write transaction 200 is transmitted from source side DPA 112 to targetside DPA 124. As shown in FIG. 2, DPA 124 records the write transaction200 in four streams. A first stream, referred to as a DO stream,includes new data for writing in LU B. A second stream, referred to asan DO METADATA stream, includes metadata for the write transaction, suchas an identifier, a date & time, a write size, a beginning address in LUB for writing the new data in, and a pointer to the offset in the dostream where the corresponding data is located. Similarly, a thirdstream, referred to as an UNDO stream, includes old data that wasoverwritten in LU B; and a fourth stream, referred to as an UNDOMETADATA, include an identifier, a date & time, a write size, abeginning address in LU B where data was to be overwritten, and apointer to the offset in the undo stream where the corresponding olddata is located.

In practice each of the four streams holds a plurality of writetransaction data. As write transactions are received dynamically bytarget DPA 124, they are recorded at the end of the DO stream and theend of the DO METADATA stream, prior to committing the transaction.During transaction application, when the various write transactions areapplied to LU B, prior to writing the new DO data into addresses withinthe storage system, the older data currently located in such addressesis recorded into the UNDO stream.

By recording old data, a journal entry can be used to “undo” a writetransaction. To undo a transaction, old data is read from the UNDOstream in a reverse order, from the most recent data to the oldest data,for writing into addresses within LU B. Prior to writing the UNDO datainto these addresses, the newer data residing in such addresses isrecorded in the DO stream.

The journal LU is partitioned into segments with a pre-defined size,such as 1 MB segments, with each segment identified by a counter. Thecollection of such segments forms a segment pool for the four journalingstreams described hereinabove. Each such stream is structured as anordered list of segments, into which the stream data is written, andincludes two pointers—a beginning pointer that points to the firstsegment in the list and an end pointer that points to the last segmentin the list.

According to a write direction for each stream, write transaction datais appended to the stream either at the end, for a forward direction, orat the beginning, for a backward direction. As each write transaction isreceived by DPA 124, its size is checked to determine if it can fitwithin available segments. If not, then one or more segments are chosenfrom the segment pool and appended to the stream's ordered list ofsegments.

Thereafter the DO data is written into the DO stream, and the pointer tothe appropriate first or last segment is updated. Freeing of segments inthe ordered list is performed by simply changing the beginning or theend pointer. Freed segments are returned to the segment pool for re-use.

A journal may be made of any number of streams including less than ormore than 5 streams. Often, based on the speed of the journaling andwhether the back-up is synchronous or a synchronous a fewer or greaternumber of streams may be used.

Image Access

Herein, some information is provided for conventional continuous dataprotection systems having journaling and a replication splitter whichmay be used in one or more examples is provided. A replication may setrefer to an association created between the source volume and the localand/or remote target volumes, and a consistency group may contain one ormore replication sets. A snapshot may be the difference between oneconsistent image of stored data and the next. The exact time for closingthe snapshot may determined dynamically depending on replicationpolicies and the journal of the consistency group.

In synchronous replication, each write may be a snapshot. When thesnapshot is distributed to a replica, it may be stored in the journalvolume, so that is it possible to revert to previous images by using thestored snapshots. As noted above, a splitter mirrors may write from anapplication server to LUNs being protected by the data protectionappliance. When a write is requested from the application server it maybe split and sent to the appliance using a host splitter/driver(residing in the I/O stack, below any file system and volume manager,and just above any multipath driver (such as EMC POWERPATH®), through anintelligent fabric switch, through array-based splitter, such as EMCCLARIION®.

There may be a number of image access modes. Image access may be used torestore production from the disaster recovery site, and to roll back toa previous state of the data. Image access may be also to temporarilyoperate systems from a replicated copy while maintenance work is carriedout on the production site and to fail over to the replica. When imageaccess is enabled, host applications at the copy site may be able toaccess the replica.

In virtual access, the system may create the image selected in aseparate virtual LUN within the data protection appliance. Whileperformance may be constrained by the appliance, access to thepoint-in-time image may be nearly instantaneous. The image may be usedin the same way as logged access (physical), noting that data changesare temporary and stored in the local journal. Generally, this type ofimage access is chosen because the user may not be sure which image, orpoint in time is needed. The user may access several images to conductforensics and determine which replication is required. Note that inknown systems, one cannot recover the production site from a virtualimage since the virtual image is temporary. Generally, when analysis onthe virtual image is completed, the choice is made to disable imageaccess.

If it is determined the image should be maintained, then access may bechanged to logged access using ‘roll to image.’ When disable imageaccess is disabled, the virtual LUN and all writes to it may bediscarded.

In an example of virtual access with roll image in background, thesystem first creates the image in a virtual volume managed by the dataprotection appliance to provide rapid access to the image, the same asin virtual access. Simultaneously in background, the system may roll tothe physical image. Once the system has completed this action, thevirtual volume may be discarded, and the physical volume may take itsplace. At this point, the system continues to function as if loggedimage access was initially selected. The switch from virtual to physicalmay be transparent to the servers and applications and the user may notsee any difference in access. Once this occurs, changes may be read fromthe physical volume instead of being performed by the appliance. Ifimage access is disabled, the writes to the volume while image accesswas enabled may be rolled back (undone). Then distribution to storagemay continue from the accessed image forward.

In some examples in physical logged access, the system may roll backward(or forward) to the selected snapshot (point in time). There may be adelay while the successive snapshots are applied to the replicationimage to create the selected image. The length of delay may depend onhow far the selected snapshot is from the snapshot currently beingdistributed to storage. Once the access is enabled, hosts may read datadirectly from the volume and writes may be handled through the DPA. Thehost may read the undo data of the write and the appliance may store theundo data in a logged access journal. During logged access thedistribution of snapshots from the journal to storage may be paused.When image access is disabled, writes to the volume while image accesswas enabled (tracked in the logged access journal) may be rolled back(undone). Distribution to storage may continue from the accessedsnapshot forward.

Disable image access may mean changes to the replication may bediscarded or thrown away. It may not matter what type of access wasinitiated, that is, logged or another type, or whether the image chosenwas the latest or an image back in time. Disable image accesseffectively says the work done at the disaster recovery site may nolonger be needed.

Delta Marking

A delta marker stream may contain the locations that may be differentbetween the latest I/O data which arrived to the remote side (thecurrent remote site) and the latest I/O data which arrived at the localside. In particular, the delta marking stream may include metadata ofthe differences between the source side and the target side. Forexample, every I/O reaching the data protection appliance for the source112 may be written to the delta marking stream and data is freed fromthe delta marking stream when the data safely arrives at both the sourcevolume of replication 108 and the remote journal 180 (e.g., DO stream).Specifically, during an initialization process no data may be freed fromthe delta marking stream; and only when the initialization process iscompleted and I/O data has arrived to both local storage and the remotejournal data, may be I/O data from the delta marking stream freed. Whenthe source and target are not synchronized, data may not be freed fromthe delta marking stream. The initialization process may start bymerging delta marking streams of the target and the source so that thedelta marking stream includes a list of all different locations betweenlocal and remote sites. For example, a delta marking stream at thetarget might have data too if a user has accessed an image at the targetsite.

The initialization process may create one virtual disk out of all theavailable user volumes. The virtual space may be divided into a selectednumber of portions depending upon the amount of data needed to besynchronized. A list of ‘dirty’ blocks may be read from the delta markerstream that is relevant to the area currently being synchronized toenable creation of a dirty location data structure. The system may beginsynchronizing units of data, where a unit of data is a constant amountof dirty data, e.g., a data that needs to be synchronized.

The dirty location data structure may provide a list of dirty locationuntil the amount of dirty location is equal to the unit size or untilthere is no data left. The system may begin a so-called ping pongprocess to synchronize the data. The process may transfer thedifferences between the production and replication site to the replica.

Read Signature Command

In one example, the current disclosure may enable read signatures for astorage device. A read signature may take a set of parameters and returna signature or hash value for those set of parameters. In some examples,the set of parameters may include a range. In certain examples the readsignature may include a LUN. In at least some examples, the readsignature command may include an LBA (Logical Block Address). In furtherexamples, the read signature may have different ranges of data. In someexamples, the read signature may return a hash value for the specifiedset of parameters. In most examples, comparing to signatures of a set ofdata may provide a way to determine if two sets of data are equivalent.In certain examples, the read signature may be implemented as a SCSIcommand.

In some examples, the signature may be calculated and stored with thedata. In alternative examples, multiple signatures may be stored fordata, each signature representing a different granularity of the data(i.e., there may be a signature for every 16 kb chunk as well as asignature for the megabyte chunk). In certain examples, when a write IOarrives, it may invalidate a stored signature for the location of thewrite IO. In other examples, a background process may update signaturesthat are out of date.

In one example, the read signature command may be used in initializationof a storage system. In some examples, read signature commands may beperformed on the data of the production site. In certain examples, theread signatures may be sent to the replication site. In furtherexamples, the replication site may perform read signature commands. Inat least some examples, the signatures of the replication site andproduction site may be compared to determine if the data is equivalenton the production and replication sites.

In alternative examples, the read signature may be performed on data ofdifferent block sizes. In some examples, the read signatures may be ofsize 8 kB. In other examples, the read signature size may be 16 kB. Infurther examples, the read signature size may be 1 megabyte. In stillfurther examples, the read signature command may contain offsets andlengths of the data. In still further examples, the read signaturecommands may be used to verify data during a disaster recovery. In someexamples, a read signature for a large data block may be used when themajority of the data is the same. In other examples, when the datasignatures of large data blocks on the replication and production siteare found to not be the same, smaller read signatures may be used todetermine what portion of the data is out of sync.

In further examples, IO may be occurring contemporaneously with thecomparison of the read signatures. In certain examples, when twosignatures are compared and are not equal, further steps may be taken todetermine if IO occurred to make the read signatures not be equivalent.In some examples, the signature may be used for verifying replicationintegrity. In another example, the signature may be used forverification and initialization. In other examples, either theproduction or replication site may not support the read signaturecommand. In the examples where one site does not support the command,the data may be read and the signature may be calculated.

Referring to FIG. 3, a read signature command 310 may be executed. Inthis example, the read signature command 310 includes a LUN, an LBA, anda block range, and signature granularity (i.e., a request may be madefor 1 MB of data in granularity of a signature for each 8 kB which wouldreturn 128 signatures).

In some examples, the signature command may be used to accelerate theaforementioned initialization process. In certain examples, theinitialization process may read all the locations which are marked asdirty in the delta marker stream and transfers the data to thereplication site. In most examples, before transferring the data, theinitialization process may check if the replication site already has therelevant data, if the data already exist at the replication site may notsend the data. In certain examples, the first time the systeminitialized, all the locations may be marked as dirty in the deltamarker stream. In some examples, the read signatures command may allowthe data at the production and replication site to be checked todetermine if the data is identical without actually reading the data.

Referring to FIGS. 4 and 5, a production site 505 may have LUN 520. LUN520 may have one or more sets of blocks. Read Signature command 514 maybe run (610). Signature 515 may be sent to replication site 535 (615).Read Signature command 550 may be run (620) at replication site 535.Signature 515 and the resulting signature from read signature 550 may becompared (625).

Referring to FIGS. 6 and 7, a read signature command 714 for LUN 720 ofProduction site 705 may be executed (805). Read signature 715 may besent to the remote site 735 (815). The remote site may check if readingsignature for the specific location is allowed (812). In certainexamples there may be locations in the do stream which contain data fromsessions before the current initialization, which have not yet beenapplied to the replication volumes. In these examples, for suchlocations, it may not be possible to use remote signature directly fromthe replication journal.

If signatures are allowed at the replication site for the specificlocation, a read signature command 750 for the data may be executed onthe remote site 735 (815). The signatures may be compared (820). If thesignatures are equivalent, the data is verified (822).

If the signatures are not equivalent, data may be read from theproduction site 805 and sent to do stream in journal 742 on thereplication site 735 (825). Open IOs may be flushed. In some examples,the process of verifying the locations suspected as different betweenthe production and replication may occur while new IOs arrive to theproduction LU. New, incoming, IOs 722 may be written to temporary stream744 (835). After the initialization, process is complete, the IOs intemporary stream 744 may be added to the DO stream in journal 742 (840).

Referring to FIGS. 6 and 8, there may not be enough space in the DO andUNDO streams of journal 742 to perform the initialization. In FIGS. 7and 9, system moves to a special mode of initialization, where there maynot be a consistent point in time available. The UNDO stream is erased(900). Data from the do stream is applied to the replication volumes(902). Once all data from previous sessions in do stream is applied tothe remote storage, (i.e., all data that was not written to the dostream in the current initialization process), the system may startapplying the data arriving from the initialization process directly tothe remote replication (903). IOs containing new data arrive totemporary stream (905). Periodically, the splitter marks a point in time(910). The splitter flushes all open IOs (915). All IOs associated withthe flush, i.e., IOs which completed before the flush time, may bemarked with the point-in-time (PIT) (930). The system applies new IOsfrom the temporary stream, which arrived before the flush point to thereplication volume, allowing the system to transfer the data from thetemporary stream to the replication volume while initialization processstill running.

During the initialization process the system reads the signature (940)for every location suspected as dirty, if the system is allowed to readsignature at the replication site (i.e., either all do stream dataapplied to the journal or the location is not marked in the do stream),the system reads the signature of the data at the replication site, andonly if the signature read is not allowed or signatures are notidentical the production site sends the initialization data to thereplication site (945).

In other examples, the read signatures command may enable a replicationsystem to perform a consistency check of the replication while IOscontinue to arrive from the production volume.

Referring to FIGS. 9 and 10, an integrity check is being performed andthere is dirty bitmap 1030 on production site 1005 and a dirty bitmap1055 on replication site 1035. The bitmaps, 1030 and 1055, track changesthat have occurred but may not have been entered on replication LUN1045. For example, if IO is in the DO stream of journal 1042, but hasnot been written to LUN 1045, then the replication bitmap 1055 may trackthis change. If the data has been changed on production site 1005 buthas not been sent to or entered on replication site 1035, thenproduction bitmap 1030 may track this change. In certain examples, toperform a full integrity check of a replicated volume, the system maytry to compare the data of the volume between the two sites.

A read signature 1014 is performed on the production site 1005 (1105).Signature 1015 from read signature 1014 is sent to the remote orreplication site 1035 (1110). Read signature command 1050 is read at theremote or replication site 1035 (1115). A determination is made if thesignatures are equal (1120). If the signatures are equal the process iscomplete and the system may move to checking the next set of blocks.(1122).

If the signatures are not equal, replication bitmap 1055 is examined tosee if the location corresponding to the signatures is in the bitmap1055 (1125). If it is in bitmap 1055, the process is done (1127). If itis not in bitmap 1055, an error is sent to the production site 1005(1130). Production bitmap 1030 is examined to see if the location is inproduction bitmap 1030 (1135). If it is in production bitmap 1030, theprocess is done (1129). If it is not in production site bitmap 1030, thedata may not be the same and may need to be refreshed (1140). Ifintegrity check fails, integrity check may stop with an error, or systemmay automatically read suspected area from the disk and sent it to thereplication site to fix the corruption.

In some examples, the read signature command may be executed in a coarsegranularity such as requesting a signature for a 10 MB block. In certainexamples, signatures for such a large block may be used to quicklycompare production and replication data to ensure consistency. In otherexamples, if a signature comparison for a coarse block, such as 10 MB,is not equal then that block may be divided up into sub block such as 1MB and the signature for these ten 1 MB sub blocks may be compared todetermine what portion of the 10 MB is not equivalent. In furtherexamples, this process may be repeated to identify the portions of thedata which is not equivalent. In alternative examples, each of thesesignatures may be calculated and stored with the LUN data. In otheralternative examples, this data may be recalculated on demand. In stillfurther examples, a stored signature may be recalculated automaticallywhen a write is sent to a particular area of the LUN.

FIG. 11 illustrates a production site 120 and a replication site 1235with signatures of different sizes for different data sizes on LUN 1210.LUN 1210 has 1 megabyte (MB) block 1215. For block 1215 LUN 1210 hasread signature 1220. LUN 1210 may also be partitioned to 16 kB blocks1225 which are part of block 1215. Each 16 kB block 1225 may havesignature 1230. LUN 1245 has 1 megabyte block 1255. For block 1255 LUN1245 has read signature 1260. LUN 1245 may also be partitioned to 16 kBblock 1265 which is part of block 1215. Each 16 kB block 1265 has readsignature 1270. In FIG. 12, these signatures are stored on production1205 and replication 1235 sites, the signatures may also be calculatedon demand when the read signature command arrives. If a comparison isperformed, read signatures 1220 and 1260 may be compared to determine ifthe blocks are equivalent. If the blocks are not equivalent, signatures1230 and 1270 may be compared to determine if blocks 1225 and 1265 areequivalent.

Write Signature Command

In one example, the current disclosure may also enable write signaturesfor a storage device. A write signature command may include a set ofparameters. For example, the write signature command may include one ormore of the following parameters:

-   -   1. LBA (logical block address) for the target of the write    -   2. Number of blocks of the IO    -   3. Signature granularity    -   4. 16 or 32 byte of hash value (or signature), for each        signature granularity block (e.g., if granularity is 16 blocks        and 64 blocks are written, then there will be 4 signatures each        of 16 or 32 bytes).

The write signature or the hash value is the same value that would havebeen returned if a read signature command was sent for the data the userwanted to write to the LBA. The write signature or hash value size(e.g., 16 byte or 32 byte or any other size) is large enough so thatthere is very high probability that if two sets of data have the samesignature then the two sets of data are identical. The purpose of thewrite signature command is to save significant amounts of bandwidth whenreplicating data from one site to another site.

In one example, a write signature command (having parameters LUN A,offset B, 16 blocks, 16 blocks granularity, signature X) means that thestorage array will search to determine if there is a data 16 blocks insize stored anywhere, with signature equal to X. If the answer is yes,the storage array will copy the data it found into LUN A offset B, andreturn a success status message. If the answer is negative, the storagearray will return a signature mismatch status, or any other failurestatus.

In one example, the write signature command may be considered as a formof remote extended copy command (xcopy), i.e., to allow copying datafrom one storage array to another storage array, without real datamovement (and without the arrays knowing of each other in this case).

Referring to FIG. 12, a storage array 1280 includes a LU A 1282 a, a LUB 1282 b and a hash table 1284. The storage array 1280 may implement thewrite signature commands as follows. In one example, the storage array1280 may allow write signature commands only for constant block sizes,e.g., allow write signature only for 8 kB aligned blocks. The storagearray 1280 may also implement the hash table 1284. In one example, thesize of the hash table 1284 may be configurable and may be large enoughto store a signature for each signature block element of the storage. Inone particular example, if the storage is 8 TB and each signaturerepresents 8 kB, 1,000,000,000 signatures would be stored each about 32bytes in size (including the metadata of where the data is beingstored). In this particular example, the hash table would only consume32 GB of data.

The key to the hash table 1284 will be the signature or hash value 1286.The hash value 1286 in the hash table 1284 will be the offset, andlogical unit, where data with the same signature is held.

The hash table 1284 may be stored on memory, on an EFD (enterprise flashdrive), on a flash card, on standard drives and so forth. The hash table1284 may be synchronously updated (i.e., with every new write the hashtable 1284 will be updated). In this case if a signature is found in thehash table 1284 the data may be immediately copied from the locationindicated in the hash table to the location the write signature commandindicated. However, a more desired implementation would be to update thehash table 1284 asynchronously. If the hash table 1284 is not completelyupdated, when a user issues a write command, the signature is searchedin the hash table 1284. If the signature is not found a signaturemismatch message will returned. Otherwise, if the signature is found,the data in the location indicated by the hash table will be read andthe signature for the data will be calculated. Only if the signaturesmatch will the data be written to the location indicated by the writesignature command and a success message will be returned. Otherwise asignature mismatch will occur.

In the case of the asynchronous update, a background process will rescanthe location in the disk that changed and update the data in the hashtable in a background process.

Referring to FIG. 13A, a process 1300 is an example of a process toupdate write signatures synchronously. Process 1300 receives a write atthe storage array 1280 (1302) and reads a signature for the offset ofthe write (1304). Process 1300 invalidates the old signature (1306)(i.e., ensuring the old signature does not point to the offset indicatedby the write) determines a hash value for the write (1308) and updatesthe hash table 1284 (1310).

Referring to FIG. 13B, a process 1350 is an example of a process toupdate write signatures asynchronously. Process 1350 marks a writelocation in a bitmap as dirty (1354) and reads the data from thelocation marked dirty in the bit map (1356). Process 1350 calculates asignature and adds the signature for the hash table 1284.

As used herein, in a hash table the key for a hash entry is thesignature. A value for the key in the hash table is the signature andthe offset and logical unit of where the data is stored.

Process 1350 waits a period of time and repeats processing blocks 1356,1358, 1360 and 1362 by reading the data from location marked dirty(1356), calculating a signature (1358), adding the signature for thehash table (1360) and waiting a period of time (1362).

Referring to FIG. 14A, a process 1400 is an example of a process tohandle write signatures synchronously. When a write signature commandarrives at the storage array process 1400 searches for data in a hashtable 1284 (1402) and determines whether the hash value is found (1404).If the hash value is found, process 1400 copies the data from thelocation indicated by the hash table 1284 to a location indicated by thewrite signature command (1406). If the hash value is not found, process1400 returns a mismatch message.

Referring to FIG. 14B, a process 1450 is an example of a process tohandle write signatures asynchronously. A write signature arrives at thestorage array and process 1400 searches for hash value in a hash table1284 (1402) and determines whether the hash value is found (1454). Ifthe hash value is found, process 1400 determines whether the hash valueis identical to the hash value of the data which is currently stored atthe location indicated by the hash table (1456). If the hash value isidentical, process 1450 copies the data from the location indicated bythe hash table 1284 to a location indicated by the write signaturecommand (1460). If the data is not found or is not identical, process1450 returns a mismatch message. Because of the way write signatures arehandled when signatures are updated asynchronously, there is no need toinvalidate old signatures.

Referring to FIG. 15, in some examples, the write signature command maybe implemented by an external data protection appliance (DPA) and not bythe storage array itself. For example, an external DPA 1470 includes ahash table 1472 and is coupled to a storage array 1480 by a fiberchannel 1474. The storage array 1480 includes a LU A 1482 a and a LU B1482 b. The external DPA 1470 may expose a virtual volume (e.g., avirtual LU A 1484 a and a virtual LU B 1484 b) for each volume in thestorage array 1480. In some examples, the virtual device (e.g., thevirtual LU A 1484 a and the virtual LU B 1484 b) will support writesignature commands and if the signature is found, the external DPA 1470will write the data to the location in the original storage array 1480.In some examples, the DPA 1470 may be a device serving write signaturecommands to the storage array 1480.

Referring to FIG. 16, a write signature command 1490 may be executed. Inthis example, the write signature command 1490 includes a LUN, an LBA(logical block address) and a block range, and signature granularity(i.e., a request may be made for 1 MB of data in granularity of asignature for each 8 kB which would include 128 signatures).

In some examples, the write signature command may be used to acceleratethe aforementioned initialization process. In certain examples, theinitialization process may read all the locations which are marked asdirty in the delta marker stream and transfer the data to thereplication site. In most examples, before transferring the data, theinitialization process may check if the replication site already has therelevant data stored somewhere on the replication storage. If the dataalready exists at the replication site, then the data may not be sent.In certain examples, the first time the system is initialized, all thelocations may be marked as dirty in the delta marker stream. In someexamples, the write signatures command may allow the data at theproduction and replication site to be checked to determine if the datais identical without actually reading the data at the production siteand transferring the data to the replication site.

In one example, a fresh installation of an application (e.g., MICROSOFT®SHAREPOINT®) is installed on a new production LUN. A new replication LUNis defined. If the application already installed on any otherreplication LUN, a significant amount of data transferring time may besaved by copying data blocks from the replication installation of theapplication instead of the production one. Thus, the write signaturesaves bandwidth.

Referring to FIGS. 17A and 18, a process 1500 is an example of a processto synchronize production and replication storage. For example, atransfer of the data is saved from the production site to thereplication site. If a user desires to send 16 blocks of data from LUN Xon a production site at offset y to the same offset at LUN Z on thereplication site, instead of reading the data and sending it to thereplication site, the signature of the data of at the LUN X offset ywith 16 block is read from the production site and sent to the replicasite. The system will try to write the signature with the signature itread from LUN X to offset y at LUN Z at the replication site.

If the replication site already has data with the same signature thenthe data is not actually read from the production site and thereplication site. Otherwise data at LUN X is read and sent to LUN Z.

The process 1500 reads the signature 1614 from LUN 1620 at a productionsite 1605 (1508) and sends the signature 1615 to the replication site1635 (1516). Process 1500 writes the read signature 1614 to the replicaLUN 1645 at replication site 535 using write signature command 1650(1522) and sends a status message to the production site 1605 whetherthe write was successful or not (1528).

If the write was successful, initialization is moved forward (1542)e.g., the area attempting to be synchronized is already synchronized.For example, if the write signature command is successful (i.e., thedata is found with the same signature at the replication site and thedata is written to the volume 1645). Thus there is no need to read thedata from the production site.

If the write was not successful, data is read from the production LUN1620 (1552) and sent to the replication site 1635 (1556), where the datais written to the same offset as the offset read at the production site.

Referring to FIG. 17B, in some other examples, a snapshot shipping modemay be used for asynchronous replication, i.e., taking an array basedsnapshot at the production site, and shipping the changes from the lastsnapshot taken at the production site and the last snapshot sent to thereplication site. The changes may be sent using the write signaturecommand. In one example, a process 1560 may include generating a firstsnapshot at the production site (1562) and tracking all changesoccurring at the production site (e.g., using a delta marking stream orany other method) (1564). Process 1560 performs an initial full sweepfrom the first snapshot to the replication site (1566) and erases thefirst snapshot (1568). Process 1560 generates a second snapshot at theproduction site (1570) and transfers the changes from the productionsite to the replication site (i.e., the changes which occurred from thetime right before creation of the first snapshot) (1572). Process 1560renames the second snapshot to be the first snapshot and repeatsprocessing blocks 1568 to 1564. In one example, each session of sendingall the changes from the production site to the replication site may usethe write signature mechanism, and if the data written to the productionvolume already exists somewhere on the replication volume no data willneed to be transferred from the production volume to the replicationvolume.

Referring to FIG. 19, a process 1700 is an example of a process toleverage a storage system supporting write signature as a device thatdeduplicated data over a network (e.g., a wide area network (WAN)). Forexample, a set of data is sent from a site A (a production site) to asite B (a replication site) and the site B includes a storage supportingwrite signatures. Process 1700 generates a signature associated withdata to be sent from a site A to a site B (1702), and sends thesignature to the site B (1708). When the signature arrives at site B,process 1700 attempts to write the signature to a location (e.g., lastoffset) in the storage volume at site B (1718) and determines whetherwriting the signature was successful or not (1724).

If writing the signature was successful, the process 1700 reads data(1728) from the location it just tried to write the signature to, andsends a status message to indicate success (1736) (i.e., the dataarrived at the replication site successfully).

If writing the signature is not successful, process 1700 sends a messageto a local site indicating that writing the signature failed (1742). Theoriginal data the production site wanted to send to the replication siteis sent to the replication site (1748). When data arrives to thereplication site it is written to the last free offset of thereplication site storage (so that the signature will be available thenext time). Process 1700 advances to the next offset (1758). In oneexample, the deduplication may be implemented by having one storage LUNat the replication site serve as the data LUN. When a data LUN is fulldata may be written again from the beginning of the LUN.

In some other examples, there may be a storage supporting writesignature commands at both a local and a remote site. The system mayknow if the signature arrived to the remote storage by checking if thesignature is available at the local storage, thus avoiding a secondround trip if the signature is not available.

Referring to FIG. 20, a computer 1900 includes a processor 1902, avolatile memory 1904, a non-volatile memory 1906 (e.g., hard disk) and auser interface (UI) 1908 (e.g., a mouse, a keyboard, a display, touchscreen and so forth). The non-volatile memory 1906 stores computerinstructions 1914, an operating system 1916 and data 1918. In oneexample, the computer instructions 1914 are executed by the processor1902 out of volatile memory 1904 to perform all or part of the processesdescribed herein (e.g., processes 1300, 1350, 1400, 1450, 1500, 1560 and1700).

The processes described herein (e.g., processes 1300, 1350, 1400, 1450,1500, 1560 and 1700) are not limited to use with the hardware andsoftware of FIG. 19; they may find applicability in any computing orprocessing environment and with any type of machine or set of machinesthat is capable of running a computer program. The processes describedherein may be implemented in hardware, software, or a combination of thetwo. The processes described herein may be implemented in computerprograms executed on programmable computers/machines that each includesa processor, a storage medium or other article of manufacture that isreadable by the processor (including volatile and non-volatile memoryand/or storage elements), at least one input device, and one or moreoutput devices. Program code may be applied to data entered using aninput device to perform any of the processes described herein and togenerate output information.

The system may be implemented, at least in part, via a computer programproduct, (e.g., in a machine-readable storage device), for execution by,or to control the operation of, data processing apparatus (e.g., aprogrammable processor, a computer, or multiple computers)). Each suchprogram may be implemented in a high level procedural or object-orientedprogramming language to communicate with a computer system. However, theprograms may be implemented in assembly or machine language. Thelanguage may be a compiled or an interpreted language and it may bedeployed in any form, including as a stand-alone program or as a module,component, subroutine, or other unit suitable for use in a computingenvironment. A computer program may be deployed to be executed on onecomputer or on multiple computers at one site or distributed acrossmultiple sites and interconnected by a communication network. A computerprogram may be stored on a storage medium or device (e.g., CD-ROM, harddisk, or magnetic diskette) that is readable by a general or specialpurpose programmable computer for configuring and operating the computerwhen the storage medium or device is read by the computer to perform theprocesses described herein. The processes described herein may also beimplemented as a machine-readable storage medium, configured with acomputer program, where upon execution, instructions in the computerprogram cause the computer to operate in accordance with the processes.

The processes described herein are not limited to the specific examplesdescribed. For example, the processes 1300, 1350, 1400, 1450, 1500, 1560and 1700 are not limited to the specific processing order of FIGS. 13A,13B, 14A, 14B, 17A, 17B and 19 respectively. Rather, any of theprocessing blocks of FIGS. 13A, 13B, 14A, 14B, 17A, 17B and 19 may bere-ordered, combined or removed, performed in parallel or in serial, asnecessary, to achieve the results set forth above.

The processing blocks (for example, in processes 1300, 1350, 1400, 1450,1500, 1560 and 1700) associated with implementing the system may beperformed by one or more programmable processors executing one or morecomputer programs to perform the functions of the system. All or part ofthe system may be implemented as, special purpose logic circuitry (e.g.,an FPGA (field-programmable gate array) and/or an ASIC(application-specific integrated circuit)).

Elements of different embodiments described herein may be combined toform other embodiments not specifically set forth above. Otherembodiments not specifically described herein are also within the scopeof the following claims.

What is claimed is:
 1. A method, comprising: generating a writesignature command, the write signature command having parameters used inexecution comprising an offset, a number of blocks of data and asignature, the write signature command configured to: write thesignature to the offset in a storage array without the data, thesignature being a hash value generated using a hash function on thedata; and enable the storage array to write the data with the samesignature to a volume if the data is available at the storage array;executing the write signature command to synchronize data between asecond site and a first site, executing the write signature commandcomprising: reading a first hash value having a first offset at thefirst site associated with a block of data at the first site; sendingthe first hash value to the second site; using the write signaturecommand with the first hash value received from the first site todetermine if there is a block of data stored at the second siteassociated with the first hash value, wherein the write signaturecommand uses the first hash value to copy a block of data, from alocation at the second site indicated in a hash table associated withthe first hash value, to a second offset at the second site if the firsthash value is found in the hash table; determining if the writesignature command was successful by determining there is the block ofdata stored at the second site associated with the first hash value; andif the write command is not successful: reading the block of data fromthe first site associated with the first hash value; and writing theblock of data from the first site associated with the first hash valueto the second offset at the second site, the second offset being equalto the first offset.
 2. The method of claim 1, further comprisingsynchronizing data between a first site and a second site withoutsending the data from the first site to the second site if the secondsite already contains data from first site located in any location inthe storage array.
 3. The method of claim 1, further comprising:receiving a write at the storage array; reading a signature for thewrite; invalidating a previous hash table entry for the signature;determining a hash value for the write; and updating the hash table. 4.The method of claim 3, further comprising: receiving a write signaturecommand containing a hash value, searching for hash value in the hashtable; copying the data from the location indicated by the hash table toa location indicated by the write signature command if the hash value isfound in the hash table; and returning a mismatch message if the data isnot found in the hash table.
 5. The method of claim 1, furthercomprising: intercepting a write command; marking a write in a bitmap asdirty; reading the data from offset indicated as dirty in the bit map;calculating a signature for the data; and adding the signature, for ahash table indicating the signature points to a write offset.
 6. Themethod of claim 5, further comprising: intercepting a write signaturecommand; searching for the hash value in the hash table; copying thedata from the location indicated by the hash table to a locationindicated by the write signature command if the hash value is found andthe hash value of the data indicated by the hash table is identical tothe hash value; and returning a mismatch message if the data is notfound or is not identical.
 7. The method of claim 1, further comprisingusing the write signature command to save bandwidth when transferringdeduplicated data over a network.
 8. The method of claim 7 wherein usingthe write signature command to save bandwidth when transferringdeduplicated data over a network comprises: generating, at the firstsite, the first signature associated with data to be sent to the secondsite; sending the first signature to the second site; attempting towrite the first signature to a location at a storage LUN at the secondsite; reading the data from the location at the storage LUN at thesecond site if the attempt to write the first signature was successful;sending a status message indicating success if the attempting to writethe first signature was successful; sending a status message indicatingfailure if the attempting to write the first signature was unsuccessful;sending the data from the first site to the second site and writing thedata to an offset at the second site if the attempting to write thefirst signature was unsuccessful.
 9. The method of claim 1, furthercomprising saving bandwidth in a snapshot shipping replicationenvironment.
 10. The method of claim 9 wherein saving bandwidth in asnapshot shipping replication environment comprises: generating a firstsnapshot at a first site; tracking changes occurring at the first site;performing an initial sweep from the first snapshot to the second site;erasing the first snapshot; generating a second snapshot at the firstsite; transferring the changes from the first site to the second site;and renaming the second snapshot to be the first snapshot, wheretransferring the changes comprises: reading signatures of the changedlocations; writing signatures to a volume at the replica site using thesignatures read at the first site; and on failure reading the data fromthe location at the first site and writing the data to the changedlocations at the replica site.
 11. An article comprising: anon-transitory machine-readable medium that stores executableinstructions, the instructions causing a machine to: generate a writesignature command, the write signature command having parameters used inexecution comprising an offset, a number of blocks of data and asignature, the write signature command configured to: write thesignature to the offset in a storage array without the data, thesignature being a hash value generated using a hash function on thedata; and enable the storage array to write the data with the samesignature to a volume if the data is available at the storage array;execute the write signature command to synchronize data between a secondsite and a first site, the instructions causing a machine to execute thewrite signature command comprising instructions causing a machine to:read a first hash value having a first offset at the first siteassociated with a block of data at the first site; send the first hashvalue to the second site; use the write signature command with the firsthash value received from the first site to determine if there is a blockof data stored at the second site associated with the first hash value,wherein the write signature command uses the first hash value to copy ablock of data, from a location at the second site indicated in a hashtable associated with the first hash value, to a second offset at thesecond site if the first hash value is found in the hash table;determine if the write signature command was successful by determiningthere is the block of data stored at the second site associated with thefirst hash value; and if the write command is not successful: read theblock of data from the first site associated with the first hash value;and write the block of data from the first site associated with thefirst hash value to the second offset at the second site, the secondoffset being equal to the first offset.
 12. The article of claim 11,further comprising instructions causing the machine to synchronize databetween a first site and a second site without sending the data from thefirst site to the second site if the second site already contains datafrom first site located in any location in the storage array.
 13. Thearticle of claim 11, further comprising instructions causing the machineto: receive a write at the storage array; read a signature for thewrite; invalidate a previous hash table entry for the signature;determine a hash value for the write; and update the hash table.
 14. Anapparatus, comprising: hardware circuitry to generate a write signaturecommand, the write signature command configured to: generate a writesignature command, the write signature command having parameters used inexecution comprising an offset, a number of blocks of data and asignature, the write signature command configured to: write thesignature to the offset in a storage array without the data, thesignature being a hash value generated using a hash function on thedata; and enable the storage array to write the data with the samesignature to a volume if the data is available at the storage array;execute the write signature command to synchronize data between a secondsite and a first site, the circuitry configured to execute the writesignature command comprising circuitry configured to: read a first hashvalue having a first offset at the first site associated with a block ofdata at the first site; send the first hash value to the second site;use the write signature command with the first hash value received fromthe first site to determine if there is a block of data stored at thesecond site associated with the first hash value, wherein the writesignature command uses the first hash value to copy a block of data,from a location at the second site indicated in a hash table associatedwith the first hash value, to a second offset at the second site if thefirst hash value is found in the hash table; determine if the writesignature command was successful by determining there is the block ofdata stored at the second site associated with the first hash value; andif the write command is not successful: read the block of data from thefirst site associated with the first hash value; and write the block ofdata from the first site associated with the first hash value to thesecond offset at the second site, the second offset being equal to thefirst offset.
 15. The apparatus of claim 14 wherein the circuitrycomprises at least one of a processor, a memory, programmable logic andlogic gates.
 16. The apparatus of claim 14, further comprising circuitryto synchronize data between a first site and a second site withoutsending the data from the first site to the second site if the secondsite already contains data from first site located in any location inthe storage array.
 17. The apparatus of claim 14, further comprisingcircuitry to: receive a write at the storage array; read a signature forthe write; invalidate a previous hash table entry for the signature;determine a hash value for the write; and update the hash table.